Don’t index files with fallback build settings

[ahoppen]

Fallback build settings don’t even have an indexstore path set, so we would never pick up any index data generated from them. Also, indexing with fallback args has some other problems:

• If it did generate a unit file, we would consider the file’s index up-to-date even if the compiler arguments change, which basically means that we wouldn’t get any up-to-date-index even when we do get build settings for the file.
• It’s unlikely that the index from a single file with fallback arguments will be very useful as it can’t tie into the rest of the project.

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test

[ahoppen]

@swift-ci Please test Windows

[ahoppen]

@swift-ci Please test Windows